Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Most engines operate efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Most automatic transmissions include planetary gear sets that modify the speed ratio between the input and output shafts. A simple planetary gear set has a sun gear, a ring gear, planet gears meshing between the sun gear and the ring gear, and a planet carrier connected to each of the planet gears. Clutches are used to couple select components of the planetary gear sets with other components to create the various gear ratios of the transmission.
Multi-disk wet clutches are commonly used to hold the components of the planetary gear sets. The clutches include clutch packs having a set of separator plates interleaved with a set of friction plates. A friction material is applied to the faces of the friction plates. The friction plates are typically connected to a rotatable component, and the separator plates are typically connected to either a rotatable or a stationary component. In many cases, a hydraulic piston is used to frictionally engage the plates to lock the components to each other when the clutch is ON.
When the clutch is ON, the friction between the plates creates enough thermal energy to damage the clutch pack if not controlled. Oil, e.g., transmission fluid, is supplied to the clutch pack to cool the clutch pack. Typically, the oil is continuously supplied to the clutch pack despite only being needed when the clutch is ON. When the clutch is OFF, the oil creates drag losses between the plates due to viscous shear loss associated with the oil. The drag losses reduce efficiency of the transmission, which can reduce fuel economy of the vehicle.
Prior patents, such as U.S. Pat. No. 6,244,407, have proposed to selectively interrupt oil flow to the clutch when the clutch is OFF to reduce drag losses between the disks and plates.